A triac, a bidirectional device with a control terminal, is commonly used as a rectifier in power electronics. The triac dimmer circuit is now widely applied in incandescent lamps and halogen lamps. The triac dimmer changes a sine wave shaped voltage such that the output voltage is kept substantially zero as long as the sine wave shaped voltage is below a target level. For example, when the sine wave shaped voltage goes below the target level of zero volts, the triac dimmer circuit does not conduct and blocks the sine wave shaped voltage. After the sine wave shaped voltage has increased to a level above the target level, the triac dimmer circuit conducts, and the output voltage is substantially identical to the input voltage. As soon as the input voltage reaches its next zero crossing, the triac dimmer circuit blocks the input voltage again. Thus, during a first part of each half period of the sine wave, the output voltage is zero. At a target phase angle of the sine wave shaped voltage, the output voltage substantially instantaneously switches to a level corresponding to the sine wave shaped voltage. By controlling the phase angle of the triac dimmer, the triac dimmer achieves light dimming.
To apply a triac dimmer in a switching mode power supply such as a light emitting diode (“LED”) driver, a bleeder dummy load is needed to maintain a minimum conducting current in the triac dimmer and to reduce LC resonance. LEDs are generally energy-saving devices, but the dummy load reduces the overall efficiency.